Glycosyltransferases have been found to be elevated in the sera of cancer patients. A unique galactosyltransferase isoenzyme was discovered in the sera of colon cancer patients and differences in fucosyltransferase isoenzymes have also been found in AML patients. Therefore we have begun investigating sialyltransferase isoenzymes in the sera of mammary tumor patients. These studies should provide evidence concerning the tissue origin of the elevated sialyltransferase activity and its role in malignancy. Together with these characterization studies of serum sialyltransferase we have attempted to synthesize specific inhibitors of sialyltransferase (Klohs, Bernacki and Korytnyk, Cancer Res. 39, 1231, 1979). Several CMP and CMP-sialic acid analogs have manifested enzyme inhibitory activity and one compound, CMP-ribodialdehyde, (Korytnyk, Angelino, Klohs and Bernacki, Eur. J. Med. Chem., in press, 1980) has shown some chemotherapeutic potential. These studies will provide more information on serum sialyltransferase in malignancy and may lead to a sensitive diagnostic or prognostic test for mammary cancer. In addition specific inhibitors of sialyltransferase may become useful for assessing this enzyme's role in normal and malignant cell functions.